pre-preprocess Moose args in constructor

[Anonymous Monk]

Esteemed and hallowed Monks: Trying to force upper case on Moose args taken from command line - eg FX currency codes - e.g.

$lookup usd eur gbp chf

Yes - I could put it into the calling code - but how would I get Moose to force it itself - I tried this from Moose docs:

trigger => sub { $_[0]->code( uc( $_[0]->code ) );

but I get message that I am unable to to modify read-only value despite Moose docs saying that trigger can modify RO values. What is correct way to force upper case (and thus in general) 'pre-preocess' object constructor arguments? Thank-you.

[haj]

There are several ways to do this, depending on how much validation you want to add. A lightweight method without any validation is to add a BUILDARGS routine to your class (see Moose::Manual::Construction):

sub BUILDARGS {
    my $class = shift;
    my %args = ref $_[0] ?  %{$_[0]} : @_;
    $args{code} = uc $args{code} if exists $args{code};
    return \%args;
}

A more idiomatic method is to add a Moose type for the currency code, check for its validity, and then allow lowercase input by converting it under the hood by Moose coercion. Here's a complete example:
Read more... See the code (896 Bytes)
BTW: I don't think that triggers are supposed to be allowed to change readonly attributes. Where did you read that?